Hi,I've been googling around but I couldn't find a satisfactory answer....

I have 5 LEDs set up in parallel being powered by a 12v/3A regulated supply (eventually to be controlled by an arduino through a TIP120 transistor)...the LEDs have a Vf of 3.5 and draw 35mA...an LED calculator online told me to use 270ohm, 1W resistors. When I power everything up...the resistors get pretty hot to the touch after a few minutes. This is a circuit that will be on for up to 8 hours at a time. Is this okay to have the resistors be so hot? I know that's what they do....but it almost seems like they might melt...or burn something else....

here is a little drawing of the circuit: http://dl.dropbox.com/u/7072449/led_circuit.jpg

It's OK for the resistors to get hot as long as they are reasonably well ventilated, not too close to each other, and not too close to anything else that objects to the heat. Each one dissipates only about 1/3 W so they are well within their rating.

However, as you don't control the LEDs individually in that circuit, it would be better to connect 2 or even 3 of them in series (with an appropriate resistor), thereby reducing both the total power consumption and the heat generated. Alternatively, if the choice of power supply is up to you, redesign it for a supply of about 6v.

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So each resistor is dropping 8.5vdc @ 35ma. That is .3 watts, so a 1 watt rated resistor should have no problem. You might look at your placement of the resistors, spacing between them, mounted a bit off PCB surface for decent air path, etc.

That of course assumes all the data you provided is accurate, including that they are indeed 1 watt rated resistors.

If you can't resolve the resistor problem, and depending on the situtaion add a small fan to help dissipate heat, in the long run it helps the life of the leds long, because while 8 hours in winter is never the problem its those 105 degree days in summer where there actually may be a hazard to the temperature of the device, life of leds, and general not catching fire

Are you certain your resistors are 270ohm (red/mauve/brown) and your power supply is 12 volts, since they should not be getting "hot" irrespective of the LED rating. And of course, is it wired as you have shown.

A 12 V source , 270 ohms getting hot, and a LED....All the resistors are hot ? 270 ohm is a bit low for a 12 V circuit. I will choose a 910 ohms or 1000 ohms. That will be a 10 mA across the resistors and the LED, assuming the LED is 2 V drop type. What purpose the LED's have ? Just to show a LED on , 10 mA is enough. I have a DIY AM Radio and the power indicator is a LED. it have a 680 ohms , the V is about 13.5 V. The resistor is a 1/4 W. I just touch it... just a "little" warm. That it. So if your resistors are THAT hot, well something is "wrong" here....